A desperate move, since a draw or defeat would seal the same fate in the match. It doesn't make much difference, since now White simply has to resign.
28... b6a6  29. a4b3  29... a6b6 Firouzja only needed a draw, so I don't think he tried too hard to find a winning line, which he certainly would be able to do if necessary. 30. b3a4  30... b6a6 